The NTSB emphasized that the FAA has been "very cooperative" throughout the investigation, including providing access to the certification review that the FAA has undertaken.

At today's on-the-record news briefing, Hersman said investigators are still "weeks away" from determining exactly what happened to cause the battery problems.

United is the only U.S. carrier flying the Boeing 787s, which have been touted as the planes of the future. However, several operated by overseas airlines have run into recent trouble, the latest because of a possible battery fire on a 787 in Japan.